With advances in speedup and density growth of optical disc devices, an optical servo for maintaining a focal point of a laser beam on an information recording track is rapidly desired to improve its precision. Especially, with speedup of an optical disc device, fluctuation in the track position that is synchronized with disc rotation, such as decentering or surface wobbling of an optical disc, increase more and more, and therefore, development of an optical disc device that can follow the fluctuation is demanded.
However, since a servo signal has a characteristic that only its frequency increases with its amplitude being maintained as the rpm of the disc increases, a higher loop gain is required to maintain a servo residual error at a specified value or lower. On the other hand, increasing the loop gain has a limitation due to a restriction such as secondary resonance of a pickup, and consequently, followability is undesirably degraded.
So, as a technique for securing the followability, a repetitive control device as disclosed in Japanese Published Patent Application No. Hei. 9-50303 (Patent Document 1) has attracted attention.
FIG. 8 is a block diagram for explaining a servo signal processing of a conventional optical disc device.
In FIG. 8, a servo signal processing system of the conventional optical disc device comprises an adder 0, a servo filter 1, an adder 2, a DAC 3, a driver 4, a pickup 5, a filter 6, a memory 7, a memory controller 8, a rotation angle detector 9, and a gain 10. A repetitive control device 20 comprises the adder 2, the filter 6, the memory 7, and the gain 10 as shown in FIG. 8.
The adder 0 detects a positional error of a focal point of a light beam with respect to a track position, and outputs a detected signal as a servo error signal to the servo filter 1. The servo filter 1 performs PID control filtering or the like on the inputted servo error signal, and outputs a generated compensation signal S3 to the repetitive control device 20.
The adder 2 adds an output S2 from the gain 10 to the output S3 from the servo filter 1 to generate a driving signal S1. The DAC 3 is a DA converter which converts the driving signal S1 outputted from the adder 2 into an analog signal. The driver 4 receives the output of the DAC 3, and generates an actuator driving current. The pickup 5 is controlled on the basis of the actuator driving signal, and moves the position of a lens to move the position of the focal point of the light beam on the optical disc.
The filter 6 is a filter of the repetitive control device 20, and generates a signal of a predetermined frequency band, which performs repetitive control. The filter 6 comprises a low-pass filter for removing noises and signals that exceed the control bandwidth of the repetitive control device 20 from the signal outputted from the adder 2, and a high-pass filter for removing DC components.
The memory 7 has plural memory areas, and signal information for one rotation of the disc is divided to be stored in the respective memory areas. The memory controller 8 changes the address of the memory 7 on the basis of an operation timing signal outputted from the rotation angle detector 9, and sends the contents of the selected address to the adder 2 as well as stores the output of the filter 6 into the memory area indicated by the address.
The rotation angle detector 9 generates a clock signal having a frequency that is phase-synchronized with an inputted spindle FG signal and is equal to an integral multiple of the spindle FG signal, and outputs the clock signal as an operation timing signal for memory control.
The gain 10 is a gain element β which multiplies the output from the memory 7 by a value not larger than 1 and outputs the resultant to the adder 2, and the gain 10 always multiplies the output of the memory 7 by a value of β≦1 so as to prevent the degree of learning from becoming 100%, thereby to satisfy the stability condition of the repetitive control.
FIG. 9 is a block diagram for explaining the memory 7 of the repetitive control device 20 shown in FIG. 8.
In FIG. 9, an arrow shown beside an optical disc 100 indicates the rotation direction of the optical disc 100, and numerals 1˜16 on the optical disc 100 indicate areas obtained by dividing the optical disc 100 into 16 in the circumferential direction.
A pickup 5 traces the tracks on the rotating optical disc 100 in the circumference direction. This is identical to the pickup 5 shown in FIG. 8.
A memory 102 has sixteen memory areas corresponding to the areas 1˜16 on the optical disc 100, and corresponding addresses are determined for the respective memory areas. A selector 103 selects one address from among the addresses of the memory 102, and writes the data from the filter 6 into the address, and further, outputs the data read from the address to the adder 2 shown in FIG. 8. The memory 7 shown in FIG. 8 is obtained by combining the memory 102 and the selector 103.
A disc motor 104 rotates the optical disc 100, and outputs an FG pulse from an FG circuit that is provided therein.
A PLL 105 multiplies the FG pulse outputted from the disc motor 104 to generate a clock signal. In FIG. 9, since the optical disc is divided into 16 areas, 16 pulses of clocks are generated for one rotation. A counter 106 counts the clocks generated by the PLL 105 to output counts from 1 to 16. The rotation angle detector 9 shown in FIG. 8 is obtained by combining the PLL 105 and the counter 106.
A memory controller 8 discriminates an area on the optical disc 100 which is currently reproduced by the pickup 101, on the basis of the count output from the counter 106, and sends the corresponding address of the memory 102 to the selector 103. It is identical to the memory controller 8 shown in FIG. 8.
FIG. 10 is a waveform diagram for explaining the manner of data updation of the memory 7. The output of the memory 7 is changed according to the memory address outputted from the memory controller 8, and the output S2 from the memory 7 via the gain 10 and the output S3 from the servo filter 1 are added by the adder 2. Thereafter, the addition result S1 is sent to the DAC 3 and, simultaneously, inputted to the memory 7 via the filter 6.
Since, for simplification, FIG. 10 is drawn with the output S3 of the servo filter 1 being zero, the output S2 of the memory 7 via the gain 10 is equal to the input S1 of the filter 6.
Next, the operation of the conventional optical disc device will be described.
Initially, a description will be given of a servo loop processing to be performed by a servo loop comprising the adder 0, the servo filter 1, the adder 2, the DAC 3, the driver 4, and the pickup 5 shown in FIG. 8.
First of all, on the basis of the position of the focal point of the light beam outputted from the pickup 5 while the pickup 5 traces the track on the rotating optical disc, and the track position that is externally inputted, the adder 0 detects a positional error of the focal point of the light beam with respect to the track position, and inputs the positional error to the servo filter 1.
In the servo filter 1, the inputted positional error signal of the light beam focal point to the track position is subjected to processings such as phase compensation, low-pass compensation and the like. Thereafter, an actuator driving signal that follows track decentering, surface wobbling or the like is outputted from the driver 4 through the servo filter 1, the adder 2, and the DAC 3 to the pickup 5. The position of the pickup 5 is appropriately controlled on the basis of the actuator driving signal outputted from the driver 4, whereby the focal point of the optical beam is maintained on the track.
Next, a description will be given of the processing of the repetitive control device 20 that performs feedforward control for the above-mentioned servo loop processing.
The signal S1 outputted from the adder 2 is input to the filter 6, wherein noises and signals that exceed the control bandwidth of the repetitive control device 20 are removed by the low-pass filter, and simultaneously, the DC components are removed by the high-pass filter.
The signal outputted from the filter 6 is stored in a predetermined address area in the memory 7, under control of the memory controller 8 that operates on the basis of the operation timing signal outputted from the rotation angle detector 9. Simultaneously, the memory controller 8 outputs the data stored in the predetermined address area of the memory 7 to the gain 10, and the signal S2 that is multiplied by a value not larger than 1 by the gain 10 is outputted to the adder 2. In this construction, a signal corresponding to one rotation of the disc, which is restricted to a predetermined frequency band by the filter 6, is stored into the divided plural memory areas of the memory 7, and sequentially outputted from the memory 7.
Thereafter, the output signal S2 from the gain 10 is added to the compensated signal S3 outputted from the servo filter 1, by the adder 2.
During the operation of the repetitive control device 20, the low-pass filter of the filter 6 functions as an anti-alias filter when the driving signal S1 is stored in the memory 7 with the address switching frequency of the memory 7 being a sampling frequency.
As described above, according to the conventional optical disc device provided with the repetitive control device 20, the signal S2 synchronized with the rotation of the disc is supplied as a feedforward signal from the memory 7 through the gain 10, whereby the signal level of the frequency component of the compensated signal S3 outputted from the servo filter 1, which frequency component is synchronized with the disc rotation, can be reduced. This is nothing but a reduction in the level of the servo error signal to be inputted to the servo filter 1, and it means that the followability to the track is enhanced.
Further, with respect to the servo characteristics, as shown in a gain diagram of FIG. 11, the loop gain at a frequency that is an integral multiple of the frequency for one rotation of the disc is increased. As the result, it is possible to enhance the followability of the pickup 5 to the fluctuation in the track position that is caused by the disc shape such as decentering or surface wobbling. FIG. 11 is a diagram illustrating the characteristics of the transfer function G of an element comprising the filter 6 and the memory 7. Among the gain characteristics, a dotted line (1) shows the characteristics of the transfer function G in the case where repetitive control is not carried out while a solid line (2) indicates the characteristics of the transfer function G in the case where repetitive control is carried out.